Optical fiber cables have been in use for communications for some time. Because of the use of glass fibers as the communications medium instead of metallic conductors, it becomes necessary to include strength members in the cables. The cables which are in use have been provided by a number of different manufacturers, each having its own structural arrangement.
Present cable structures fall into two major classes, a loose tube arrangement and a loose fiber bundle arrangement. In a typical loose-tube arrangement, a maximum of fibers are packaged in an individual buffer tube, and several tubes are helically stranded together over a central member to form a core. In some optical fiber cables, strength members are disposed centrally of the cable cross section. See, for example, German Offenlegungsschrift 25 51 210 in which a plurality of individual tubes are arrayed about a central strength member. In such an arrangement, it is typical for the length of the optical fiber generally to equal the length of the tube and excess fiber length is provided by the radial position of the fibers in the helically stranded buffer tube. This construction inherently is space inefficient and results in relatively large and heavy cables.
On the other hand, the loose fiber bundle arrangement includes a plurality of unstranded fiber bundles disposed in a single core tube which extend in a direction along the center line of the cable without stranding. This arrangement which does not include a central strength member and in which strength members are disposed outside the core tube is better suited for low-fiber-count optimization than is the loose tube arrangement. A cable having a metallic armor layer and being based on the loose fiber bundle approach, is known in the art; however, it is limited to a single bundle which includes only twelve fibers.
In the loose fiber bundle approach, the strength members are included in a sheath system of the cable. As an example of the latter arrangement, see U.S. Pat. No. 4,826,278 which was issued on May 2, 1989 in the names of C. H. Gartside, III, A. J. Panuska, and P. D. Patel. Therein, at least one optical fiber bundle is disposed within a tubular member which may be made of polyvinyl chloride (PVC). The tubular member is enclosed in sheath system elements such as a water blocking tape and an outer jacket. Longitudinally extending strength members may be disposed in the outer jacket, generally adjacent to the tubular member.
It is not uncommon to have cables include a layer of metallic armor. Although such armor is used to provide rodent and lightning protection, it also cooperates with the jacket and longitudinally extending strength members to provide strength properties for the cable.
It also is not unusual to provide optical fiber cables with excess length optical fibers, that is, optical fibers each having a length which exceeds the length of the cable. This is done so that when the cables are handled and routed in tortuous paths during installation, undue strains will not be induced in the fibers. The excess length of fiber generally has been accomplished by applying controlled forces to the cable before takeup to stretch elastically the cable sheath components, allowing excess fiber lengths to become disposed in each increment of length of the cable as the stretching forces are released and the sheath components return to their unstretched length.
In the prior art, there has been reluctance to use a relatively high excess fiber length. The reason for such reluctance is the concern that too much excess fiber length results in undesired bending of fibers which leads to increased attenuation. As a result of the typically low excess fiber length, the strength member system which may be supplemented by a metallic armor layer must be sufficient to limit the stresses being imparted to the optical fibers. The strength member system must be such that any elongation of the cable under load will not exceed the safe strain limit of the optical fibers.
Customer input and requests have revealed that an economical, lightweight cable optimized for a maximum of twenty-four fibers is needed for predominantly aerial applications. To date, most engineering attention has been given to optical fiber cables with relatively high fiber counts used in point-to-point telephony applications. A relatively high percentage of optical fiber cable sales, however, have consisted historically of relatively low fiber counts, that is, no more than twenty-four fibers. A number of low fiber count cables are presently offered in the marketplace, but the structural arrangements are similar to those used in cables having a relatively high fiber count. Therefore, a need exists both in telephony and in specialty markets for an economical cable design optimized for low fiber counts. What is sought after is an economical cable that is relatively small in diameter and relatively low in weight yet one which retains excellent optical performance and which has a 600 lb tensile load rating. These requirements seemingly are at odds with the requirements for strength to prevent damage to the optical fibers.
What is needed and what seemingly is not available in the prior art is a cable having a limited number of optical fibers and which is relatively light in weight. Desirably, such a cable is relatively low in cost and can be made with existing manufacturing equipment.